RFC: OneOf Input Objects

[benjie]

First came the @oneField directive.

Then there was the Tagged type.

Introducing: OneOf Input Objects and OneOf Fields.

OneOf Input Objects are a special variant of Input Objects where the type system asserts that exactly one of the fields must be set and non-null, all others being omitted. This is represented in introspection with the __Type.oneField: Boolean field, and in SDL via the @oneOf directive on the input object.

OneOf Fields are a special variant of Object Type fields where the type system asserts that exactly one of the field's arguments must be set and non-null, all others being omitted. This is represented in introspection with the __Field.oneArgument: Boolean! field, and in SDL via the @oneOf directive on the field.

(Why a directive? See the FAQ below.)

This variant introduces a form of input polymorphism to GraphQL. For example, the following PetInput input object lets you choose between a number of potential input types:

input PetInput @oneOf {
  cat: CatInput
  dog: DogInput
  fish: FishInput
}

input CatInput { name: String!, numberOfLives: Int }
input DogInput { name: String!, wagsTail: Boolean }
input FishInput { name: String!, bodyLengthInMm: Int }

type Mutation {
  addPet(pet: PetInput!): Pet
}

Previously you may have had a situation where you had multiple ways to locate a user:

type Query {
  user(id: ID!): User
  userByEmail(email: String!): User
  userByUsername(username: String!): User
  userByRegistrationNumber(registrationNumber: Int!): User
}

with OneOf Input Objects you can now express this via a single field without loss of type safety:

input UserBy @oneOf {
  id: ID
  email: String
  username: String
  registrationNumber: Int
}
type Query {
  user(by: UserBy!): User
}

FAQ

Why is this a directive?

It's not. Well, not really - its an internal property of the type that's exposed through introspection - much in the same way that deprecation is. It just happens to be that after I analysed a number of potential syntaxes (including keywords and alternative syntax) I've found that the directive approach is the least invasive (all current GraphQL parsers can already parse it!) and none of the alternative syntaxes sufficiently justified the increased complexity they would introduce.

Why is this a good approach?

This approach, as a small change to existing types, is the easiest to adopt of any of the solutions we came up with to the InputUnion problem. It's also more powerful in that it allows additional types to be part of the "input union" - in fact any valid input type is allowed: input objects, scalars, enums, and lists of the same. Further it can be used on top of existing GraphQL tooling, so it can be adopted much sooner. Finally it's very explicit, so doesn't suffer the issues that "duck typed" input unions could face.

Why did you go full circle via the tagged type?

When the @oneField directive was proposed some members of the community felt that augmenting the behaviour of existing types might not be the best approach, so the Tagged type was born. (We also researched a lot of other approaches too.) However, the Tagged type brought with it a lot of complexity and controversy, and the Input Unions Working Group decided that we should revisit the simpler approach again. This time around I'm a lot better versed in writing spec edits 😁

Why are all the fields nullable? Shouldn't they be non-nullable?

To make this change minimally invasive I wanted:

• to make it so that existing GraphQL clients could still validate queries against a oneOf-enabled GraphQL schema (if the fields were non-nullable the clients would think the query was invalid because it didn't supply enough data)
• to allow existing GraphQL implementations to change as little code as possible

To accomplish this, we add the "exactly one value, and that value is non-null" as a validation rule that runs after all the existing validation rules - it's an additive change.

Can this allow a field to accept both a scalar and an object?

Yes!

input FindUserBy @oneOf {
  id: ID
  organizationAndRegistrationNumber: OrganizationAndRegistrationNumberInput
}

input OrganizationAndRegistrationNumberInput {
  organizationId: ID!
  registrationNumber: Int!
}

type Query {
  findUser(by: FindUserBy!): User
}

Can I use existing GraphQL clients to issue requests to OneOf-enabled schemas?

Yes - so long as you stick to the rules of one field / one argument manually - note that GraphQL already differentiates between a field not being supplied and a field being supplied with the value null.

Without explicit client support you may lose a little type safety, but all major GraphQL clients can already speak this language. Given this nonsense schema:

input FooBy @oneOf {
  id: ID
  str1: String
  str2: String
}
type Query {
  foo(by: FooBy!): String
}

the following are valid queries that you could issue from existing GraphQL clients:

• {foo(by:{id: "..."})}
• {foo(by:{str1: "..."})}
• {foo(by:{str2: "..."})}
• query Foo($by: FooBy!) {foo(by: $by)}

If my input object has only one field, should I use @oneOf?

Doing so would preserve your option value - making a OneOf Input Object into a regular Input Object is a non-breaking change (the reverse is a breaking change). In the case of having one field on your type changing it from oneOf (and nullable) to regular and non-null is a non-breaking change (the reverse is also true in this degenerate case). The two Example types below are effectively equivalent - both require that value is supplied with a non-null int:

input Example @oneOf {
  value: Int
}

input Example {
  value: Int!
}

Can we expand @oneOf to output types to allow for unions of objects, interfaces, scalars, enums and lists; potentially replacing the union type?

🤫 👀 😉

[wyfo]

Is @oneOf missing in the example of section Can this allow a field to accept both a scalar and an object?

[wyattjoh]

I’d worry that statements around type safety are a little hard to apply in practice.

It’s not the case typically that a directive would change a types underlying type yet @oneOf seems to imply that “when the server gets this, it expect one of these to be non-null”. Off the bat, a standard GraphQL to Typescript conversion could do something like

type PetInput = { cat?: CatInput; dog?: DogInput; fish?: FishInput; }

When instead I’d expect it to do something like:

type PetInput = { cat: CatInput; } | { dog: DogInput; } | { fish: FishInput; };

I totally understand the motivation around the change to make it as low impact as possible, but I'd worry about the adverse side affects introduced by this subtle change to the ways that the null/non-null properties are determined.

Maybe I’m just applying my understanding incorrectly, but I’d hope that any adoption doesn’t in fact mutate the type system of GraphQL using directives like this.

[benjie]

@wyfo Thanks, fixed!

@wyattjoh It’s not a directive, it’s a new type system constraint that DOES model the type of the input differently and would have different types generated. Have a look at the alternative syntaxes document for other ways this could be exposed via SDL and let us know your preference, perhaps you would prefer the oneof keyword to make it clearer (in SDL only, this would not affect introspection) the change in behaviour.

[cometkim]

It looks like an existing syntax, but the semantics are different? I am worried that if it will end up asking for dirty exception handling for every directive code path.

Have a look at the alternative syntaxes document for other ways this could be exposed via SDL and let us know your preference

Could we consider a new syntax that hasn't been mentioned?

type  Query {
   user(id: ID!): User
   user(email: String!): User
   user(username: String!): User 
   user(registrationNumber: Int!): User
}

pros?:

• it might be easy to apply because it just releases the existing constraints (that field names cannot duplicate on SDL)
• it makes the schema can look intuitive for the possible input type.

cons:

• Conversely, it makes it look like a variant is possible for the output.

[benjie]

@cometkim Can you show how that syntax would be expanded to input objects too, please? And yes we can absolutely consider alternative syntaxes.

[wyfo]

It’s not a directive

Why should it be something else than a directive?

Actually, it's already (almost) possible to implement @oneOf as a directive in a few lines of code.
I've made a Gist to show a possible implementation using Python and graphql-core (quite the reference translation of graphql-js in Python).
In fact, if the field directive is trivial, the input type directive requires in my example a graphql-core specific feature. However, the proposal of input object validation (still opened) could bring the material needed to implement it with graphql-js.

By the way, GraphQL schema is kind of poor in validation stuff (compared to JSON schema for example), so part of the validation is already done by the resolvers/scalar parsing methods. In a schema-first approach, you can also defines directives for repetitive checks, maybe with JSON schema-like annotations, but your code/library will have to translate and inject them into your resolvers/scalar types(/input types when the mentioned proposal will pass).
IMO, @oneOf should not be different as a directive, it could just be a validation marker used to add validation code in the resolvers/input type; no need of the type system validation. Also, in a code-first approach (no directives), it's already possible to support tagged unions, I do it in my own library; there is no need of the SDL.

In fact, I don't really see the interest of making @oneOf something else than a validation directive. And I'm wondering then if a validation directive would be appropriate in the GraphQL specification … Maybe it could be a kind of convention for schema-first libraries. Yet, having it in the specifications could help tooling (linters, code generators) and lighten GraphQL libraries. Anyway, night thoughts.

[sungam3r]

Can we expand @OneOf to output types to allow for unions of objects, interfaces, scalars, enums and lists; potentially replacing the union type?

For input types @oneOf implies one more nesting level. What do you think @oneOf will look like for unions?

[sungam3r]

Or oneArg to inline with args ?

[sungam3r]

Is the word literal appropriate here in case of using variables? The same question about Oneof for Input Object.

[benjie]

I believe so; I've modeled it on the language already used in this section, namely: https://spec.graphql.org/draft/#sel-LALTHHDHFFFJDAAACDJ-3S

[sungam3r]

It’s not a directive, it’s a new type system constraint that DOES model...

@benjie I don't understand. You wrote about @oneOf as a directive in the spec and at the same type talk here that it's not a directive... 😕

[benjie]

For input types @OneOf implies one more nesting level. What do you think @OneOf will look like for unions?

Another nesting level; i.e. instead of querying like:

{
  allEntities {
    ... on User { username }
    ... on Pet { name }
    ... on Car { registrationNumber }
    ... on Building { numberOfFloors }
  }
}

it'd look like:

{
  allEntities {
    user { username }
    pet { name }
    car { registrationNumber }
    building { numberOfFloors }
  }
}

[benjie]

@benjie I don't understand. You wrote about @OneOf as a directive in the spec and at the same type talk here that it's not a directive... confused

The input union working group have not decided what syntax to use for oneOf yet. It might end up as being presented as a directive, or it might be a keyword or any other combination of things. Check out this document for alternatives: https://gist.github.com/benjie/5e7324c64f42dd818b9c3ac2a91b6b12 and note that whichever alternative you pick only affects the IDL, it does not affect the functionality or appearance of GraphQL operations, validation, execution, etc. Please see the FAQ above.

TL;DR: do not judge the functionality of this RFC by its current IDL syntax. We can change the IDL syntax.

[sungam3r]

It might end up as being presented as a directive

OK. In my opinion if something is presented as a directive than ... it is just a directive.

[benjie]

Thanks for the review @sungam3r; good to have additional scrutiny! I don't think any modifications to the RFC are required to address your concerns (other than perhaps writing an alternative IDL syntax, but I don't plan to invest time in that until there's general concensus on what the syntax should be, for now the directive syntax can act as a placeholder). I think all the conversations in your review can be closed except for the oneArg suggestion; that one might require some more bike-shedding 😉

[eapache]

Missing { a: $varA, b: $varB } with various combinations of values for varA and varB.

[leebyron]

My in meeting proposal was that this case could just be invalid at start.

This L1441 in Validation file in this PR sounds like it would do just that:
https://github.com/graphql/graphql-spec/pull/825/files#diff-607ee7e6b71821eecadde7d92451b978e8a75e23d596150950799dc5f8afa43eR1441

[benjie]

These are exactly the same as for input objects (which also don't specify what happens if you have multiple variables); but I'll add some for clarity.

[benjie]

@leebyron Good catch; that was not my intent. I have updated the PR with better validation and more examples.

[benjie]

I've since revisited my thoughts on this and for the sake of defining types of variables on the client I've adopted the suggestion: #825 (comment)

[aleksandarsusnjar]

@benjie

We discussed an approach like you describe above, @aleksandarsusnjar, and decided that moving things down a level would achieve the same goal without requiring the added complexity ...
...
... but the directive was seen as the simplest.

Yep, I agree. I was just trying to illustrate, not propose an alternative, at least not yet.

@tobiasdiez:

one sees that the @OneOf proposal is (logically) equivalent to an input union with the constraints

1. Every type involved in the union has to have exactly one field.
2. These fields have to have different names.

Not sure what you're trying to point out here. (1) isn't a constraint of @oneOf but that of a traditional type union, mapped to fields. (2) is a standard constraint of pretty much anything, in any language.

The names of the required fields have to uniquely identify one of the types involved in the union.

I disagree with that. I illustrated it to help readers connect the dots but @oneOf does not require this and benefits from that. It can accomplish more without that. Note that @oneOf is not a type union and, I'd argue, it should not aspire to be one. It just addresses many present pain points for which one might think of type unions but in a simpler and different way.

[tobiasdiez]

@tobiasdiez:

one sees that the @OneOf proposal is (logically) equivalent to an input union with the constraints

1. Every type involved in the union has to have exactly one field.
2. These fields have to have different names.

Not sure what you're trying to point out here. (1) isn't a constraint of @oneOf but that of a traditional type union, mapped to fields.

It's a constraint that comes from the mapping to fields.
For example, { kitten: Kitten, dog: Dog } | { human: Human } is a valid type union, but mapped to fields it would read something like { animal: { kitten: Kitten, dog: Dog }, human: Human }. I argue above that there is no intrinsic need for the introduction of the additional nested animal (since both types involved in the union can be distinguished).

(2) is a standard constraint of pretty much anything, in any language.

The constraint is across all the fields in the union types. For example, it is not possible to have the type union { kitten: Kitten, age: int} | {dog: Dog, age: int} being mapped (via the above mapping) to a @oneOf type. You could write it as something like { kitten: { kitten: Kitten, age: int}, dog: {dog: Dog, age: int}.

It can accomplish more without that. Note that @oneOf is not a type union and, I'd argue, it should not aspire to be one.

I tried to show above that there is a 1-1-correspondence between @oneOf and certain type unions. In some ways it's just a different syntax, but with the added constraints 1 + 2. I think these constraints can be relaxed, but not easily so in the @oneOf syntax (you actually tried to do exactly that in #825 (comment)), but more easily in the input type syntax. So, almost by definition, the oneof can only accomplish less than the more flexible type union.

[benjie]

The constraint is across all the fields in the union types. For example, it is not possible to have the type union { kitten: Kitten, age: int} | {dog: Dog, age: int} being mapped (via the above mapping) to a @OneOf type. You could write it as something like { kitten: { kitten: Kitten, age: int}, dog: {dog: Dog, age: int}.

Indeed, if all fields are nullable then there's no way to discern what {age:3} means. I think you'd write it something like this? (I changed age which seems like it would be a property of Kitten/Dog to a duration of ownership instead, which makes sense to live outside of the animal model.)

input AnimalOwnership {
  animal: Animal!
  durationInDays: Int
  # ...
}
input Animal @oneOf {
  cat: Cat
  dog: Dog
}

[tobiasdiez]

The constraint is across all the fields in the union types. For example, it is not possible to have the type union { kitten: Kitten, age: int} | {dog: Dog, age: int} being mapped (via the above mapping) to a @OneOf type. You could write it as something like { kitten: { kitten: Kitten, age: int}, dog: {dog: Dog, age: int}.

Indeed, if all fields are nullable then there's no way to discern what {age:3} means. I think you'd write it something like this? (I changed age which seems like it would be a property of Kitten/Dog to a duration of ownership instead, which makes sense to live outside of the animal model.)

input AnimalOwnership {
  animal: Animal!
  durationInDays: Int
  # ...
}
input Animal @oneOf {
  cat: Cat
  dog: Dog
}

Yes, that would work but doesn't let you reuse CatOwnership { cat: Cat!, durationInDays: Int} (and a similar DogOwnership) in case you use these in the rest of the schema.

All I wanted was to point out that typed unions are essentially equivalent, in the sense that everything you can model using on of the construction can be achieved in the other construction as well:

• Every @oneOf construction can be mapped to a typed union:

@oneOf { a: A, b: B, ...} -> {a: A!} | {b: B!} | ...

• Every typed union can be mapped to a @oneOf construction by introducing dummy fields

  { a_1: A_1, a_2: A_2, ...} | { b_1: B_1, b_2: B_2, ...} | ... -> @oneOf { a: { a_1: A_1, a_2: A_2, ...}, b: { b_1: B_1, b_2: B_2, ...}
  ̀ ``
  
  

Although equivalent, I think the typed union yields easier to understand code and less nesting.

[yaacovCR]

I wonder if these lines relating to variable usage pre-empt the discussion around #1059 and should be pulled from this spec change (simplifying it).

Variables must be only of the allowed type, but it seems that we should specify what that entails for all variables and types only in one place, i.e. the separate rule.

So if we currently allow variables of nullable types to be used in non-null positions and throw a field error at runtime -- which we do -we should continue to do so irrespective of isOneOf, and if/when we make the change there, that should be done in a way that covers isOneOf as well.

Encountered this while attempting to rebase graphql/graphql-js#3813

[yaacovCR]

There are two arguments against this:

1. The benefit of the current relaxed version of VariablesInAllowedPositions is that you can use a variable for an argument, not supply the value, and get the default. But with OneOf, default values are not allowed for any fields, so treating this as the same as the general case would only be sensical if (a) we adopt the strict version of the general rule (b) we can convince ourselves that there is a real value in consistency almost for consistency's case.
2. We can only consider this to be a specific case of the general rule if the @oneof directive is held to transform all of the input object's field types into non-nullable (but still not required) types. Then, these become non-nullable positions. There is a certain ambiguity as to whether the field types themselves are nullable or not. By syntax, we want to make sure older clients can leave them out, and so we define them to be nullable. But for clients aware of @oneof, presumably we are ok to define them as non-nullable, with the caveat that there would have to be a change to the IsRequiredInputField algorithm. Currently, an input object field is required if is of a non-null type and does not have a default value. This would have to be changed to have an additional condition, that the parent input object does not have isOneOf to be true. Note that IsRequiredInputField is part of graphql-js as a utility, and referenced many times in the spec, but does not form a formal agorithm.

[benjie]

I wonder if these lines relating to variable usage pre-empt the discussion around #1059 and should be pulled from this spec change (simplifying it).

I don't think so? Technically all fields on a oneof are nullable, but you must specify one and it must be non-null, so this seems a very straightforward way to require that when it comes to a variable? #1059 handles non-null positions, but this is a nullable position according to the type system.

[yaacovCR]

and it must be non-null

So it's a nullable type only because we want introspection to say that it's nullable because currently that's the only way of making something optional. But a null cannot be supplied, so in a sense it's by definition "a non-nullable position."

So we would then have to introduce the concept of non-nullable positions that occur when (1) the type for the position is non-nullable or (2) the containing type is oneOf, and then the general rule about matching nullable and non-nullable would have to depend on this new "position" concept rather than the type itself.

As I type this, I can see that this additional layer is a bad idea, and I appreciate the compromise that you have ended up with.

On the other hand, in GraphQL 2.0 / TreeQL, we should definitely separate optionality and nullability, and remember to change oneOf to be better defined. (It really shouldn't be the case that you cannot use null at a nullable position.)

[yaacovCR]

In the working group, Lee suggested that when writing Oneof Input Object literals we should always only allow one field to be present (whether or not variables were involved). I did not want to do this because it makes oneofs more distinct from regular input objects, and it's frustrating that a user cannot do something like:

type Mutation {
  addPet(pet: PetInput): Pet
}

input CatInput { name: String!, nickname: String, meowVolume: Int }
input DogInput { name: String!, nickname: String, barkVolume: Int }

input PetInput @oneOf {
  cat: CatInput
  dog: DogInput
}

#----

mutation AddPet ($cat: CatInput, $dog: DogInput) {
  addPet(pet: {cat: $cat, dog: $dog}) { name }
}

This feels like a natural thing to do.

Until... (and this is where I've come around to Lee's suggestion) until you want to apply strong typing to the variables on the client side. Now you have to analyse every usage of the variables in the client in order to determine that the type of the variables is something like (TypeScript syntax): { cat: CatInput! } | { dog: DogInput! }. It's technically feasible, but it's a tonne of effort for the client, when previously you could just look at the variable definitions and you were done.

For that reason, I think I'm going to adopt Lee's suggested approach. One additional benefit of Lee's approach is it's easier to implement the validation rules for it. Further we can open it up in future should we wish without breaking any previously valid queries - so it's a decision that we can revisit later.

If you want stronger typing, can't you do:

mutation AddPet ($pet: PetInput) {
  addPet(pet: $petInput) { name }
}

Although I agree that the stricter initial approach is best, and we could always relax later!

[benjie]

If you want stronger typing, can't you do:

Isn't that exactly what Lee's proposing? Requiring people to pass oneOf types directly, or single concrete pre-resolved types ({ cat: $nonNullableCat }). It just forbids the loose typing of { cat: $nullableCat, dog: $nullableDog }.

[yaacovCR]

If you want stronger typing, can't you do:

Isn't that exactly what Lee's proposing? Requiring people to pass oneOf types directly, or single concrete pre-resolved types ({ cat: $nonNullableCat }). It just forbids the loose typing of { cat: $nullableCat, dog: $nullableDog }.

What I’m arguing is that it doesn’t make sense to adopt @leebyron suggestion forbidding weaker typing in order to allow for stronger typing if stronger typing is possible regardless with better variable definitions. I mean, I see why someone might disagree with the above , but just to clarify what I’m saying.

[benjie]

Oh right; I see what you're saying. I agree with Lee though, let's guide people to the pit of success 👍

[YutaUra]

LGTM

[t1]

Please excuse me, if this is a noob question, but I didn't quite understand, why it would be such a bad idea to allow @oneOf on (the parameters of) a method?

The examples given above seem a bit over-complicated to me. What I'd like to define, is something like this:

input CatInput { name: String!, nickname: String, meowVolume: Int }
input DogInput { name: String!, nickname: String, barkVolume: Int }

type Mutation {
  addPet(cat: CatInput, dog: DocInput): Pet @oneOf
}

#----

mutation AddPet ($cat: CatInput, $dog: DogInput) {
  addPet(cat: $cat, dog: $dog) { name }
}

On the client side, the @oneOf validation can be done from the schema of the addPet mutation. This would then forward to the AddPet operation.

[jbellenger]

why it would be such a bad idea to allow @oneOf on (the parameters of) a method?

input CatInput { name: String!, nickname: String, meowVolume: Int }
input DogInput { name: String!, nickname: String, barkVolume: Int }

type Mutation {
  addPet(cat: CatInput, dog: DocInput): Pet @oneOf
}

I think the example you give indicates the problem: you've tried to apply @oneOf to the collection of arguments but have instead invalidly applied it to the Pet type.

The GraphQL grammar does not afford applying directives to multiple argument types (see TypeSystemDirectiveLocation, the closest you can get is ARGUMENT_DEFINITION which describes a single argument type). Extending directives to allow this would be counter to the goal of a proposal that is easy to implement.

[t1]

I was very unsure about the allowed locations of directive, so I looked it up in the spec (draft version): a directive is allowed on a FIELD_DEFINITION, i.e. a simplified version of example No 91:

directive @example on FIELD_DEFINITION

type SomeType {
  field(arg: Int): String @example
}

[benjie]

why it would be such a bad idea to allow @oneOf on (the parameters of) a method?

I don't think it would be "such a bad idea", but it does open a can of worms. For example, consider this (partial) schema:

type Query {
  userById(id: ID!): User
  userByUsername(username: String!): User
  userByRegistrationNumber(registrationNumber: String!): User
}

In the current proposal you could collapse this to something like:

input UserFinder @oneOf {
  id: ID
  username: String
  registrationNumber: String
}
type Query {
  user(by: UserFinder!): User
}

What you're proposing is to condense this further:

type Query {
  user(
    id: ID
    username: String
    registrationNumber: String
  ): User @oneOf
}

At first blush, this looks like a great win, right? But one issue is that it severely inhibits schema evolution. Let's imagine that you now want the ability to see what a user looks like when viewed as another user; our original schema would look like:

type Query {
  userById(id: ID!, asUser: ID): User
  userByUsername(username: String!, asUser: ID): User
  userByRegistrationNumber(registrationNumber: String!, asUser: ID): User
}

The current proposal allows a similar change:

 input UserFinder @oneOf {
   id: ID
   username: String
   registrationNumber: String
 }
 type Query {
-  user(by: UserFinder!): User
+  user(by: UserFinder!, asUser: ID): User
 }

But your proposed @oneOf on field arguments wouldn't allow for this, so we'd have to add a new field...

 type Query {
   user(
     id: ID
     username: String
     registrationNumber: String
   ): User @oneOf
+  userAsUser(by: UserFinder!, asUser: ID): User
 }
+input UserFinder @oneOf {
+  id: ID
+  username: String
+  registrationNumber: String
+}

And the structure of the new field would likely be something like Query.userAsUser(by: UserFinder!, asUser: ID): User... This is essentially the exact same solution that the current proposal supports.

So by preventing @oneOf applying to arguments, we lead users into the "pit of success" and make it so that they can easily evolve these fields in non-breaking ways in future.

We did discuss applying @oneOf to only certain arguments (@oneOf(args: ['id', 'username', 'registrationName'])) but that opens more questions: do we now support @oneOf on multiple sets of arguments? How is this expressed through introspection? What kind of validations do we need in place for this? Ultimately we decided it was too complex and easily solved by just using a oneOf input object directly.

Further, one of the GraphQL guiding principles is:

Simplicity and consistency over expressiveness and terseness
Plenty of behaviors and patterns found in other languages are intentionally absent from GraphQL. "Possible but awkward" is often favored over more complex alternatives. Simplicity (e.g. fewer concepts) is more important than expressing more sophisticated ideas or writing less.
-- https://github.com/graphql/graphql-spec/blob/main/CONTRIBUTING.md#guiding-principles

So again, since it's already achievable with input objects there doesn't seem to be a pressing need to extend it to arguments.

I hope this helps you understand why the decision was made to not apply @oneOf to arguments at this time.

[t1]

@benjie: yes, that explains it very well. I'm not sure I fully agree, but now I understand and fully accept your reasoning. Thank you!